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Old 09-16-2007, 06:14 PM
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Hit the brakes and lose the volts...?

I have an 1983 K5 Blazer (diesel original, 350SBC now w/ original harness). I have been driving this for many years now with no grief until just last weekend I went to leave the hardware store (during work of course) and went to back up out of the parking space. As soon as I depressed the brake pedal the truck died. I restarted, again it died w/ the brakes. I have no brake lights, no fuses blown. My voltmeter drops to dead bottom when the brakes are depressed and thankfully I didn't have a fire on my hands. I ran a jumper wire to the HEI temproraily to get me home. By the time I got home (20 mi later) I had no turn signals and my power windows wouldn't work either. My guess is a dead short in the harnass somewhere against perhaps some body metal or through the firewall......

I just wanted to get your two cents worth and perhaps some guidelines for an efficient and effective procedure to troubleshoot this problem. I certainly don't want to burn my truck down and I am scared with it not blowing any fuses.


Add on:

Ok, went to work on it today and behold, after sitting a week, I could not duplicate any of the issues I had last weekend. Wierd!! What else to do but field test so I put on over 10 miles in town (stop,go,turns, etc) and 30 miles on highway with no issues. Went to back up at dad's and suddenly it all came back again. Seemed like it happened when I put in reverse. matter of fact, it did it before when I put it in reverse. I have a brand new ignition module. Could the Neutral Safety switch cause this? I bought a new ignition switch just to eliminate that as it is cheap enough. No, I haven't gotten it installed yet.


Help please!!! PS. Got a fire extinguisher too!!!

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Old 09-16-2007, 06:25 PM
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have you checked the ground(s)?
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Old 09-16-2007, 06:40 PM
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Everything seems good. I have dual battery set up with each battery grounded. I tried running on Bat.1, Bat.2, and both (yes, I have an isolator to keep em seperated and use a large switch (got it from the RV supply store)). Everything comes off this switch. (whoever decided to use the junction to the starter is an idiot in my opinion). Anyways, I have been running this system for a couple of years and haven't changed anything in the truck lately. Noticed today on the way home that any switched circuit on the truck would cause the voltmeter to drop to zero. It never would rise above 10 volts or so. When I got home I checked the batteries w/ truck running (14.4-14.5 volts on both) and not running (12.9-13.0) on both. Alternator was putting out 14.9volts. Isolator had good voltage as well. I crawled under and followed the wires running to the rear of the truck. All were in a loom and clipped to the frame every 2 feet or so. No probs there. It has gotta be under the dash somewhere. I really am starting to lean towards the harnass/fusebox going **** up!!
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Old 09-16-2007, 07:25 PM
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Hmmmmm

Just an initial thought here, and this MAY be more than one problem, but with the key OFF, get under the dash and press the brake pedal down with your hand.

Look to see if the back of the metal part of the brake pedal arm touches any wires?

Look to see if any wires move whenever you press that pedal OR change gears.

How does it start? Does the starter turn over as fast as it always did?

What happens when you turn on the headlights without the engine running?

Finally, check the fuse box and see if you see anything funny, such as corrosion, funny looking stuff on the fuse box etc.

Steve
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Old 09-16-2007, 08:16 PM
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didn't check the brake pedal itself. Will do tomorrow hopefully.

same with wires against the brake pedal.

Engine starts just like always. Not a starter issue.

Did check the headlights with truck off. All good. Seem just as bright as always.

And finally, of course the fuse box looks funny, it was a diesel originally. For the most part everything is good. A few wires have beed added onto additional empty circuits for various things. Nothing new has been done. I am suspecting a problem in the back of the fusebox to firewall area. Even better my gut keeps saying that it is a bad ignition module on the column but I just replaced it with a new one about a month and a half ago. It was all fine until last weekend.

Hypothetically speaking, could the screws that adjust the ignition module location on the column back and forth have come loose allowing the module to slide away? Would that give me these symptoms? What boggles me is that I have no brake lights, truck off or on, and that should be a circuit that is hot all the time.....
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Old 09-16-2007, 09:02 PM
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Another thought

Has anyone removed the tail light assemblies recently to change a bulb?

Even if not, remove them and look to see if any wires are pinched up against the metal. It MAY not be obvious, maybe just a little black mark on the wire.

If nothing obvious there, follow the wires back up to their originating point or to the fuse box. See if they come close to the exhaust, or cross any wires that look burnt, might be pinched to the frame somewhere etc.

Steve
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Old 09-16-2007, 09:41 PM
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To start out, make sure the main instrument panel ground is connected. It is usually a black wire attached to a screw on the upper part of the emergency brake mechanism. I don't know how it would cause your specific problems, but if that ground wire has come off it can definitely make troubleshooting very difficult.

Also check the small ground wire that goes from the negative battery terminal to the fender, and the braided ground strap from the engine to the firewall. There is also a ground on the left taillight assembly.

Bruce
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Old 09-17-2007, 06:44 AM
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If you are not smelling burning wires or don't see any harnesses that look cooked; I would think it sounds like a intermittant bad power connection. Like maybe at the bulkhead connections?

It could be just coincidence about putting it in reverse; maybe those 2 extra bulbs going on just put a little more load on that bad connection?

Stuff like this is hard to trace if the problem goes away before you can trace back the circuits that go dead.
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Old 09-17-2007, 12:55 PM
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Without being able to duplicate it consistent, this is hard, but you might try disconnecting the brake light switch next time it happens. You might be able to at least rule that out.
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Old 09-17-2007, 01:41 PM
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I find it highly suspect that you would have a short without blowing a fuse. Also, the neutral switch only controls the starter circuit and you said it cranks over fine, so our problem lies elsewhere.

You said you have no brake lights at all? It is correct that it is an always hot circuit. Do the hazard flashers work? They should be on the same power circuit. If they don't work, check the fuse. Visual inspection can fail as the filament can burn out under the metal contact on each end. Take a multimeter, set it on ohms (0x001 if it is manual range) and be sure it reads 0.0-1.0 ohms. If the fuse is good, check for +12V on that circuit at the fusebox, and then check the switch. One of those two is most likely the culprit.

It sounds to me more like coincidence that the brake lights failed at the same time the engine cuts out random. Is the volt gauge the stock one in the dash? I have a wiring diagram for a 77-79 and according to that the volt gauge comes right off the ignition switch. A bad ignition switch would also explain why you lost power windows and all your switched power accessories. Take a look under the dash to make sure your ground is attached, and check to be sure you have no loose connections to the ignition switch.

Let us know what you find so we can get you on your way again.
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Old 09-18-2007, 05:36 PM
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First of all, I want to say thanx for all the responses I have gotten.

Now, on to the problem. Replaced the ignition yesterday. Nothing changed. I am narrowing it down to somewhere on the column or fuse box area. I was gonna lower the column today and check the ignition module for adjustment and any loose wires, etc. but it is raining and I have been outside in it working all day. No more for me.

I am not naive to electricity. I have done my share of wiring stuff in my times. I have traced all the wires from the cab to the rear and it is all good. I have checked all the grounds and they all seem good. I have checked all the bulbs, repalced both bulbs, both sides in the rear. front ones all seem to work fine.

A whole new wiring harnass is starting to look pretty good. I was also suspecting that maybe I got a defective new Ignition Module when I replaced it a few months ago....
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Old 09-18-2007, 08:39 PM
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Meter?

Do you have a digital volt meter? They are dirt cheap these days and even parts stores have them. Go digital though and not analog. Analog work great, but the learning curve is higher.

I'd start checking wires to ground. If you can get a wiring diagram of your truck that's going to be a big help.

Did you manually move the brake pedal and look for wires touching ground?

I think one of the things you should do next is remove the fuse box and look behind it.

Right now some things aren't making sense, but when you find the problem, it will. Something as simple as a wire or paper clip or other metal object falling behind the fuse box, or the brake pedal touching a wire but only making partial contact through the insulation...it could be ten thousand things. I have seen a lot of weird things!

I used to pride myself on fixing problems other put in hours on and then gave up. Patience, curiosity and not letting it beat me pushed me on until I found it. The rewards are incredible feelings of conquering something that beat others. The one I remember most was someone replaced a part and pinched a wire when reinstalling the part. It was not obvious and not easy to find, but i didn't let it beat me.

We're just making educated guesses still and the meter and a diagram would help. When you find it, the problem will make sense. It may not be obvious, but don't give up. Have patience and keep wondering where else to look.

On the other hand, if you think the truck needs a new wiring harness anyway, it might be a great excuse?

Steve
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Old 09-19-2007, 02:11 AM
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Quote:
I find it highly suspect that you would have a short without blowing a fuse.
Not every single wire is fused.

If it kills the engine when you hit the brakes, you have a dead short. If you replaced the ignition module then I would look there first. Double check any new wire that you have ran. I highly doubt it is the fuse block because if it was about the only thing that goes wrong with them is bad connections where the main harness plugs in on the firewall side. You either have a dead short in the area of your brake pedal that is hitting a chaffed wire, or a hot back to the brakes is not hooked up correctly. Does anything else at all act erratic when the symptom starts acting up? Have you added any relays of any type for anything?

Kevin
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Old 09-22-2007, 04:57 PM
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Ok, problem fixed. I lowered the steering column yesterday to get a better look at that ignition module. the mounting holes have slots about a 1/4 " long. I loosened the mounting bolts and shoved the module as far upward as I could, therefore giving the most actuation from the ignition switch/rod. Retightened (probably more than necessary!!) and now everything works fine again. It has been 2 days and no issues so time will tell.

Odd that the module worked fine for well over a month after installing with no issues. Guess maybe I didn't tighten the bolts well enough the first time is the only thing I can come up with. They seemed tight enough when I readjusted it the other day though......oh well. Thanx all!!
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